What thickness of board do I need?

Thickness is the wrong place to start and it is where everyone starts. The two questions that actually decide it are how far the board has to span unsupported and what has to be fixed into it — and once you have answered those, the thickness usually chooses itself.

Step by step

1. Start from the span, not the thickness.Halving the span reduces sag roughly sixteen-fold; increasing the thickness by half reduces it roughly threefold. Adding a support in the middle of a shelf does far more than going up a thickness, and it is cheaper.
2. Then ask what has to be fixed into it.A hinge, a runner, a lock or a heavy bracket needs enough board to hold a screw. Below about 15mm most sheet materials will not hold a face fixing under load, and nothing holds well in an edge below 18mm.
3. Use 3 to 6mm for backs and linings.Cabinet backs, drawer bottoms, panel linings, templates. Thin board in a rebate or a groove is structurally very effective — a 4mm back rebated into a carcass is what stops the carcass racking, and it does that job regardless of its thickness.
4. Use 9 to 12mm for small panels and light work.Small cabinet sides, drawer boxes, light shelving over short spans, sub-frames, boxing in. Under 12mm, sheet material starts to feel flexible in hand-sized panels and will not hold an edge screw.
5. Use 15 to 18mm as the general-purpose carcass thickness.This is the default for cabinets, wardrobes, shelving, doors and almost all furniture, because it is stiff enough over normal spans and thick enough to take a hinge, a runner and an edge screw. When in doubt for interior joinery, 18mm.
6. Use 18 to 22mm for floor decks and structural sheathing.Loft boarding, floor decks and roof decks are matched to the joist spacing and the load, so this is a case where you take the figure from the manufacturer's table or from building control rather than from a rule of thumb.
7. Use 25mm and up for heavy shelving and worktop substrates.Long shelves, workbench tops, worktop substrates and anything taking real weight. Note that a 25mm board weighs proportionally more, which matters for both the fixings and the carrying.
8. Consider a lipping or a frame instead of more thickness.A solid hardwood lipping on the front edge of a shelf, or a rail turned on edge under it, stiffens the board far more than an extra few millimetres of thickness would, and costs less. Depth in the direction of bending is what buys stiffness.
9. Take structural thicknesses from a table, not from a guess.Floor decks, roof decks and anything spanning between joists have published thickness-to-span requirements. Undersizing them produces a bouncy floor at best. Use the manufacturer's data and check with building control on anything notifiable.

Tips

• 12mm and 18mm cover the vast majority of domestic work. If you are stocking a workshop, those two thicknesses in a decent plywood will do almost everything.
• A thin board in a groove or a rebate performs far above its thickness because it is restrained on all edges. That is why cabinet backs are so thin and still stop the carcass twisting.
• Doubling up two thinner sheets glued together gives you a thicker panel that is often flatter and cheaper than a single thick sheet, and it lets you laminate a curve.

Common mistakes

• Going thicker to fix a sagging shelf — Span is the dominant term. A shelf sagging over 1000mm will still sag over 1000mm in a thicker board, just less. Add a support or shorten the span and the problem disappears.
• Using 12mm where hinges and runners have to go — There is not enough material for a face fixing to hold under repeated load, and none at all for an edge screw. Cabinet sides carrying hardware want 18mm.
• Choosing floor deck thickness by eye — It is a structural decision tied to joist spacing and load, with published requirements. Getting it wrong gives a floor that flexes and creaks and, on a loft, one that is not safe to load.

Questions people ask